Breast cancer (BC) is an age-associated disease, with risk increasing sharply with age, so that approximately 75% of BC cases occur in postmenopausal women (50 years and older). The majority of these cancers initially are estrogen (E)-dependent; however, breast tumors typically progress over time to become E-independent or endocrine-resistant. This progression is a complex process that is believed to occur in three stages. Considerable research has focused on the use of hormone replacement therapy (HRT) by women with E-responsive breast cancer, and most oncologists/physicians do not recommend HRT to these women. Ironically, there has been a dramatic increase in the consumption of estrogenic isoflavones, often at high levels, by postmenopausal women with BC, because these substances are perceived to be a "safe", natural alternative to HRT. The safety of the dietary estrogenic isoflavones for women with E-responsive BC, however, has not been adequately evaluated. Our preliminary results in an established animal model for BC (MCF-7 tumors in athymic mice) indicate that low dietary doses of genistein (GEN) can facilitate the progression of breast tumors to a GEN-induced E-independent (GIEI) state. If such a change were to occur in women, it might worsen prognosis and limit the effectiveness of endocrine therapy. In our proposed experiments, we will define more precisely the dose and duration of exposure of dietary GEN that causes this phenotypic change in MCF-7 tumors to progress to GIEI tumors. We will also evaluate the potential for the isoflavone metabolite equol, which is more estrogenic than its parent isoflavone daidzein, to cause a similar phenotypic change. In a separate set of experiments and in collaboration with Project 4, we will evaluate the role of ER-beta in MCF-7 cells and determine whether these weak estrogens, which bind preferentially to ER-beta and mediate ER-beta dependent transactivation, play a role in altering BC progression. We will also conduct mechanistic studies in collaboration with Project 4 and using the resources of Core C to conduct microarrays analysis to profile gene expression changes as tumors progress from E-dependent to GIEI tumors. In summary, our studies will determine potential mechanisms by which low dietary dosages of GEN or equol at physiologically relevant dietary levels can cause progression to an E-independent tumor. These preclinical studies have important human health implications in aging women, because women with E-dependent BC are consuming dietary estrogens and the safety of these estrogenic products is unknown.